When manufacturing companies use conveyors to tie their in-process machinery together, they often not only want the conveyors to move their product between machines but to also accumulate the product between machines. Accumulation on a conveyor happens when product is stopped from moving forward off the end of the conveyor and subsequent products, still moving, come to a rest when they bump into the first one stopped. A second, third, and forth, etc., product then lines up behind the first one stopped—all while the conveyor is running. Equally important then is when the stop is removed, the stopped products need to resume their forward travel. The accumulation ability of conveyors allows the manufacturer to run an asynchronous operation. If no accumulation of product were possible, due to conveyor limitations, all the machines required for the product manufacturing process would have to run at the same rate and would be locked in step, that is, synchronized. With non-accumulating conveyors, if one of the synchronized machines needed to be stopped momentarily to clear the jam, all the upstream machines would also have to be stopped. Then all those machines would have to be restarted simultaneously. Many companies avoid this situation by using accumulating conveyors to connect their machinery. This way, when any one machine goes down, the upstream machines can keep producing with their output just accumulating on the conveyor.
Accumulating conveyors came in many types. Some products require a certain type of conveyor to be used during their manufacture; some products can work with any type of accumulating conveyor. The basic goals for an accumulating conveyor include the ability to transport and accumulate the product without damaging the product.
When moving a stack of loose unbound sheets of paper some conveyor types do not accumulate without damaging the bottom layers. Others do not transport without damaging the bottom layers. When a stack of loose paper or other horizontally stacked product is conveyed on some roller conveyors, the trailing edge of the bottom layers can be displaced out the backside of the stack. This creep or shingling of the bottom-most layers will potentially cause subsequent machinery to damage the bottom displaced sheets. Some roller conveyors can accumulate stacks of paper as long as the rollers can be stopped from rotating while products are in accumulation.
When paper stacks are conveyed on a roller conveyor, the weight of the stack is concentrated on the lines of contact tangential with each roller. This produces compression zones in the lower layers of paper in the area just above each roller.
The amount of compression depends upon the type of paper and the height of the stack. The paper in the area between the rollers then forms what looks like a sag, but is really an area that is not as compressed as the zones directly over the roller. When the rollers rotate to convey the paper stack forward, the compression zones and sags do not move relative to the conveyor; rather it is the paper that moves through them. Each new roller encountered by the paper stack creates its own compression zone and resulting sag. When the trailing edge of the moving stack comes off a roller, the compression zone disappears and the extra material in the sag is seen as displaced paper. Each roller creates a sag and all the sags, worked back and off the trailing edge, contribute to the total displacement of the lowest layers. The longer the distance the stack travels on a roller conveyor, the more displacement is created. Also, the taller the stack the greater is the displacement. If this creeping or shingling is allowed to grow, it is possible that the entire stack of products can fall over.
What is needed is an apparatus and method which minimizes or eliminates shingling or creeping of products in a stack. The present invention does this in a novel and unobvious way.